Originally Posted By: BigCahuna
Rotella has the MA spec which says it's safe for use with wet clutches, which you have in a Sportster. As far as the trans goes, 10 million metric bikes whose oil is shared between the motor and trans, prove it can handle transmission use. I wouldn't use gear oil in the primary of a Sportster. While it wouldn't hurt the gears in the trans, I'd be concerned with excess drag on the clutch plates. But it's a free world.,,,
Harley recommends 20W50 for the engine and formula + for the trans and primary.and also 20W50 for the trans and primary. When I had my 1000 mile service at the dealer, they put harley Syn3 20W50 synthetic in both the engine and the primary. I have to say it runs and shifts better with the SYN3 20W50 in the trans. I have no trouble finding netural.
Now when I do my next oil change I am going to use Red Line 20W50 in the engine and 75W90 GL5 gear oil in the trans and the primary. The Red Line is yellow metal safe AND 75W90 is the SAME viscosity as 20W50 engine oil so no drag on the clutch. Red Line 75W90 GL5 gear oil has additives for clutches in limited slip diff so will work for a wet clutch.
The tect at Red Line said to me in an email they had tested the GL5 gear oil in Sportsters for a long time and thats what he recommended.